1. Field of the Invention
The present invention relates to improvement of a monolithic multiple-wavelength laser device including a plurality of laser sections formed on a single substrate, and improvement of a method of fabricating the same.
2. Description of the Background Art
Optical disks for optically recording information, such as CDs (compact disks), DVDs (digital video disks), and MDs (mini disks), are now widely used as high-capacity recording media.
In using such an optical disk, information is recorded and reproduced via an optical pickup. Depending on the difference in recording densities which vary according to the types of the optical disks, an optical pickup for reproducing a CD uses laser light of a wavelength band of about 780 nm, and an optical pickup for reproducing a DVD uses laser light of a wavelength band of about 650 nm.
In recent years, instead of optical disk devices each for reproducing a CD or a DVD exclusively, an optical disk device capable of reproducing both a CD and a DVD has been developed, using an optical pickup including a multiple-wavelength semiconductor laser device formed by integrating semiconductor laser devices of lasing wavelength bands of about 650 nm and about 780 nm.
A schematic cross sectional view of FIG. 4 shows an example of a conventional monolithic multiple-wavelength laser device. It is to be noted that, in the drawings of the present application, the same or corresponding portions are denoted by the same reference numbers. Further, in the drawings of the present application, dimensional relation among length, thickness, width, and the like is changed as appropriate to clarify and simplify the drawings, and it does not represent the actual dimensional relation.
In the monolithic multiple-wavelength laser device of FIG. 4, on an n type GaAs inclined substrate 601 having a main surface 15° off a crystallographic (001) plane toward a [110] direction, a laser section 602 for a CD and a laser section 603 for a DVD are formed in parallel.
In CD laser section 602, an n type GaAs buffer layer 604, an n type Al0.5Ga0.5As clad layer 605, an n type Al0.3Ga0.7As guide layer 606, an active layer 607, a p type Al0.3Ga0.7As guide layer 608, a p type Al0.5Ga0.5As first clad layer 609, and a p type GaAs etch stop layer 610 are formed successively. On p type GaAs etch stop layer 610, a ridge-shaped p type Al0.5Ga0.5As second clad layer 611 and a p type GaAs cap layer 612 are formed successively, and both sides of the ridge are buried in n type GaAs current blocking layers 613.
In DVD laser section 603, an n type GaAs buffer layer 614, an n type GaInP buffer layer 615, an n type (Al0.7Ga0.3)0.5In0.5P clad layer 616, an undoped (Al0.5Ga0.5)0.5In0.5P guide layer 617, an active layer 618, an undoped (Al0.5Ga0.5)0.5In0.5P guide layer 619, a p type (Al0.7Ga0.3)0.5In0.5P first clad layer 620, and a p type GaInP etch stop layer 621 are formed successively. On p type GaInP etch stop layer 621, a ridge-shaped p type (Al0.7Ga0.3)0.5In0.5P second clad layer 622, a p type GaInP intermediate layer 623, and a p type GaAs cap layer 624 are formed successively, and both sides of the ridge are buried in n type GaAs current blocking layer 613.
On p type GaAs cap layers 612 and 624 in CD laser section 602 and DVD laser section 603, respectively, a p type ohmic electrode 625 and an Mo/Au electrode 626 are formed successively. An n type ohmic electrode 627 is formed on the back side of n type GaAs substrate 601.
A laser section isolating trench 628 is formed between CD laser section 602 and DVD laser section 603 to reach substrate 601, to electrically isolate the both laser sections from each other. Each monolithic multiple-wavelength laser device is separated from a wafer along a chip separating trench 629.
The monolithic two-wavelength semiconductor laser device of FIG. 4 fabricated as described above is mounted on a sub-mount, with its n type electrode side up, and with its p type electrode side mounted on a surface of the sub-mount. Then, the sub-mount is attached on a prescribed stem.
The monolithic two-wavelength semiconductor laser device including loss guide structures utilizing GaAs buried layers 613 as shown in FIG. 4 is preferable in that the buried layers in both of the CD laser section and the DVD laser section can easily be formed simultaneously. In the loss guide structure, however, operating current of the laser device is increased due to large loss in wave guiding, significantly narrowing the allowable range of thermal design and the like in optical pickup design. Consequently, it is preferable to employ a real guide structure by using for the current blocking layer a material such as AlInP or a dielectric film which allows laser light for a CD and laser light for a DVD to pass through.
In the case that AlInP is used for the current blocking layer, however, the growth of the current blocking layer on the sides of the ridge in the CD laser section is slow, making it difficult to control growth conditions for burying the ridge completely. Further, since unnecessary AlInP layer portions having grown on the p type GaAs cap layers in the CD and DVD laser sections are not flat-shaped, it is difficult to control process conditions for removing the unnecessary portions. In the case that a dielectric film is used for the current blocking layer, thermal conductivity of the dielectric film itself is lower than that of the semiconductor crystal and thus thermal radiation efficiency is deteriorated, making it difficult to maintain high temperature characteristics and ensure reliability especially in the DVD laser section which originally does not have so good temperature characteristics.
In each of the CD and DVD laser sections, feedback light noise may be caused due to interference with light reflected from an external optical system. To reduce such noise, it is desired to employ a self-pulsation structure in each laser section. The self-pulsation structure can be implemented by adjusting a difference ΔN in effective refractive indices in and out of the ridge in a horizontal direction, and an optical confinement factor Γ in the active layer. However, design ranges of ΔN and Γ for causing self-pulsation have a trade-off relation with respect to improvement of reliability and reduction of operating current in the laser device.